1. Field of the Invention
The present invention relates to a light collecting device for a projection apparatus, and particularly, relates to a light collecting device that is easy to manufacture and adapted for a multi-light source projection apparatus.
2. Descriptions of the Related Art
In general, digital light processing (DLP) projection systems comprise many components such as a light source, a color splitter, a light condenser and a digital micromirror device (DMD), which are configured to process light beams to generate an image. In such a DLP projection system, a light collecting rod is typically disposed in the optical path immediately after the light source to integrate the light beams and improve the uniformity thereof. Light beams from the light source will enter the light collecting rod, where they are reflected multiple times before exiting an emergent surface. As a result, the light beams will be integrated and thus, exhibit a better uniformity.
All optical systems are restricted by the étendue conservation law, in which only light falling within a limited light source area and a limited light divergence angle is effective. To improve the brightness of a DLP projection system, a multi-light source illumination structure may be adopted by increasing the number of light sources to improve the brightness of displayed images. Because the limit of étendue conservation law, the added light source area and the light divergent angle must be arranged to be small enough so that the light beams can pass the aperture of the projection lens and fall within the size of the panel.
The design of the light collecting rod will become more critical when light beams from multiple light sources need to be integrated and uniformed. A multi-light source illumination structure for a projection system and a light collecting device thereof have been disclosed in the prior art, as shown in FIGS. 1A and 1B. This conventional illumination structure 1 comprises a first light source 11, a second light source 13, a first light collecting rod 15, a second light collecting rod 17 and a third light collecting rod 19. The first light source 11 is opposite to the second light source 13. The light collecting rods 15, 17, 19 have incident surfaces 151, 171, 191 and emergent surfaces 153, 173, 193, respectively. The first light collecting rod 15 and the second light collecting rod 17 further have reflecting surfaces 155, 175. A first light beam 111 from the first light source 11 enters the first light collecting rod 15 through the incident surface 151, and is reflected by the reflecting surface 155 before exiting the emergent surface 153. Similarly, a second light beam 131 from the second light source 13 enters the second light collecting rod 17 through the incident surface 171, and is reflected by the reflecting surface 175 of the second light collecting rod 17 before exiting the emergent surface 173. After exiting the emergent surfaces 153, 173, the light beams enter the third light collecting rod 19, and then exit the emergent surface 193 for projection onto other optical elements at the back-end. In this way, light beams from two different light sources may be integrated and made uniform. The overall size of the light source system is then reduced due to the elimination of condensing optics.
However, this light collecting device is difficult to manufacture due to its relatively complex structure. Because the three light collecting rods are small, it is difficult to join the first light collecting rod 15 to the second light collecting rod 17 and the third light collecting rod 19. Additionally, since these light collecting rods have poor couplings therebetween, the more times the optical path passes through these couplings, the greater the amount of lost light. Moreover, it is difficult to coat reflective materials on the tiny reflective surfaces 155, 175.
In addition, the DMD of the DLP projection system requires a specific aspect ratio (typically 4:3, 16:9 or 16:10). Restriction of the light sources themselves forces them to be arranged horizontally in the projection apparatus. Therefore, in order for the emergent surface 193 of the light collecting rod to meet this specific aspect ratio of the DMD, and in consideration of the horizontal arrangement of the light sources, additional optical components will be necessary to project the light beams on the DMD precisely. This adds cost and also unnecessarily incurs more light loss.
Moreover, the conventional light collecting device is only adapted to be used in association with two light sources, and will face a challenge if it is used in a multi-light source projection apparatus, thus, lacking flexibility in application.
In summary, although the conventional multi-light source illumination structure can be used in association with two light sources, the light collecting device thereof is difficult to manufacture and suffers from light loss when coupling. Therefore, a light collecting device, which is small, easy to manufacture, has a high light collecting efficiency and that can be used for a multi-light source projection apparatus is desired in this field.